Compliant foil-fluid bearing support arrangement

ABSTRACT

A foil-fluid bearing assembly adapted for supporting a shaft within an outer bearing housing and is adapted for accommodating angular misalignment of the rotating shaft relative to the housing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/232,391 filed Aug. 30, 2002.

TECHNICAL FIELD

The present invention relates generally to a foil-fluid bearing, and inparticular, to a foil-fluid bearing comprising compliant supports.

BACKGROUND OF THE INVENTION

Foil-air bearings are advantageous as they operate efficiently over alarge temperature range, have increasing load capacity as rotationalspeeds increase, can handle severe environmental conditions, and aremore reliable than rolling element bearings because they require fewerparts to support the rotating assemblies and do not need lubrication.However, they also require inherently tight tolerance controls on thebearings themselves and their housings in order to maintain a very closealignment between each radial bearing in a rotor system in order tooperate effectively. Bearing misalignment can be caused by excessivemanufacturing tolerances, operational loads or thermal distortion of thebearing housing, and can result in shaft angular misalignment causingunacceptable whirl instability. Existing foil-air bearings also requirea good control of both radial stiffness and damping characteristics ofthe inner foil support structure.

The use of hydrodynamic fluid journal bearings having compliant foils isknown. The inner foils of the bearing are arranged such that the endsections of the foils themselves are more compliant than the centralsection of the foils, and can deflect independently in order to provideimproved conformity to a misaligned rotor supported by the bearing.Strong misalignment forces can thereby be tolerated. U.S. Pat. No.4,274,683 teaches such a compliant foil bearing. However, thesehydrodynamic journal bearings require metallic corrugated foilassemblies that are comprised of several separate sections of foils,which must each be independently flexible and able to deflect differentcontrolled amounts. Introducing independent flexibility to the metallicfoils themselves considerably adds to the complexity, and consequentlycost, of the bearing. Additionally, as each bearing requires severaldiscrete metallic foil sections having different flexibility, thecomplexity and manufacturing costs are further increased.

There remains a need to provide a foil-fluid bearing capable oftolerating angular misalignment, without significantly adding complexityto the internal construction of the bearing, and consequently withoutsignificantly adding to material and manufacturing costs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedfoil-fluid bearing capable of increased angular misalignment tolerance.

It is an object of the present invention to provide a foil-fluid bearinghaving a simplified compliant support arrangement.

Therefore, in accordance with the present invention, there is provided afoil-fluid bearing assembly adapted for supporting a shaft within anouter bearing housing, the foil-fluid bearing assembly comprising: abearing sleeve having an outer surface and an inner circumferentialsurface diametrically sized to receive the shaft therewithin such thatan annular clearance gap is radially defined between an outer surface ofthe shaft and the inner circumferential surface, the annular clearancegap adapted for accommodating fluid therein and axially extending alonga substantial portion of a length of the bearing sleeve; a foil elementdisposed within the annular clearance gap; a compliant bearing supportcontiguously disposed between the outer surface of the bearing sleeveand the outer bearing housing, and permitting elastic deflection of thecompliant bearing support, independent of movement of the foil element;and the compliant bearing support comprising at least two independentcompliant support elements respectively having a first and a secondmodulus of elasticity, wherein the first modulus of elasticity is lessthan the second modulus of elasticity; whereby angular misalignmentcapability for the foil-fluid bearing is provided by the compliantbearing support.

There is also provided, in accordance with the present invention, afoil-fluid bearing assembly adapted for rotatably supporting a shaftwithin a bearing housing, the foil-fluid bearing assembly comprising: abearing sleeve having an outer surface and an inner surface defining aninner race, said inner race diametrically sized to receive a rotatingshaft therewithin and maintain an annular clearance gap radially betweensaid rotating shaft and said inner circumferential surface, the annularclearance gap axially extending along a substantial portion of a lengthof the bearing sleeve, the bearing sleeve adapted, in use, to supply asupport fluid to substantially fill said annular clearance gap; a foilelement circumferentially disposed between said inner race and saidshaft; a compliant bearing support disposed between the outer surface ofthe bearing sleeve and the bearing housing; and the compliant bearingsupport having at least two independent compliant support elements, saidat least two independent compliant support elements respectively havinga first and a second modulus of elasticity, wherein the first modulus ofelasticity is less than the second modulus of elasticity.

There is additionally provided, in accordance with the presentinvention, a foil-fluid bearing assembly comprising: a compliantlysupported bearing sleeve, radially disposed between a rotating shaft andan outer housing, such that an annular clearance gap, adapted forreceiving a support fluid therein, is provided between the rotatingshaft and the bearing sleeve; a foil element being radially disposedwithin the annular clearance gap; the bearing sleeve being compliantlysupported within the outer housing by at least two independent compliantsupports, radially disposed between the bearing sleeve and the outerhousing; and the two independent compliant supports respectively havinga first and second modulus of elasticity, the first modulus ofelasticity being less than the second modulus of elasticity.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is an axial cross-sectional view of a shaft supported at eitherend by a compliantly supported foil-fluid bearing according to thepresent invention.

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a partial cross-sectional view enlarged from region 3 of FIG.1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, each foil-fluid bearing assembly 10 of thepresent invention generally comprises a bearing sleeve 16 supporting arotating shaft 12 within an outer bearing housing 14. At least onecompliant support 30,32 is radially disposed between the bearing sleeveand the outer housing, and is capable of elastically deflecting totolerate misalignment of the bearing sleeve with respect to the outerhousing. The bearing sleeve 16 comprises an inner bore therethroughhaving a diameter such that a precise annular radial gap 24 is definedbetween an inner circumferential surface of the sleeve and an outersurface of the shaft. Much like all foil bearings, the present bearingcomprises an inner foil member 28, disposed within the annular radialgap 24. The foil member 28 is preferably comprised of at least twofoils, an inner smooth foil providing a fluid film running surface andat least one outer corrugated foil or equivalent spring foil. Manycorrugated foils can be radially stacked between the inner smooth foiland the inside circumferential surface 18 of the bearing sleeve 16. Thecorrugated foils provide some of the low stiffness and damping requiredfor the compliance of the smooth foil. The smooth foil provides a smoothrunning surface for the film of fluid that forms within the annularradial gap 24.

Traditional air bearings having sleeves rigidly fixed to the surroundinghousing, require very tight tolerance controls on the bearings and theirhousings, in order to maintain a good alignment between each of theradial bearings of a rotor system.

The present foil-fluid bearing preferably comprises three compliantsupports installed on each bearing sleeve, each compliant supportcontiguous with, and radially disposed between, the bearing sleeve andthe outer bearing housing. A central compliant support, axially locatedbetween two compliant end supports, having a higher modulus ofelasticity than the two end compliant supports, permits the bearingsleeve to swivel around the stiffer middle compliant support.

This permits the compliant bearing to self-align with another radialbearing supporting the shaft or rotor system. Such a shaft or rotorsystem could for example, have at least another similar compliantfoil-fluid bearing at its opposite end, as shown in FIG. 2. Similarly,the present compliant foil-fluid bearing could be used on a shaft orrotor system in combination with any number of other bearings. Thisenables greater tolerance for bearing misalignment, which can resultfrom any one of, or combination of, several factors includingmanufacturing tolerances, operational loads, and thermal displacementsof the housing.

Referring now to FIG. 2 and FIG. 3, the foil-fluid bearing assembly 10of the preferred embodiment comprises the bearing sleeve 16 comprisingan inner circumferential surface 18, and an outer circumferentialsurface 20 having three annular support points thereon. The supportpoints each comprise an annular notch, disposed radially outwardly andbeing adapted for receiving an annular compliant support therein.Specifically, the middle support point 22 has annular notch 25 formedtherein, and the two outer support points 23 define annular notches 27therein. The central support point notch 25 receives annular compliantmiddle support 32, and the two outer support points notches 27 receiveannular compliant end supports 30. As mentioned above, the two compliantend supports 30 have a predetermined lower modulus of elasticity, andare therefore less stiff, than the middle compliant support 32. Whilethe middle support point 22 is preferably at the axial midpoint of thebearing sleeve 16, the distance between the middle support point and theend support points can be modified as required to provide more or lessrotational flexibility of the bearing sleeve about an axis perpendicularto the longitudinal axis 13 of both the bearing sleeve and the shaft 12.Both the stiffer central compliant support 32 and the outer compliantsupports 30 are in contacting engagement with the inner circumferentialsurface 15 of the outer bearing housing 14.

The generally cylindrical bearing sleeve 16 comprises a concentriccentral bore therethrough, defined by the inner circumferential surface18 that is precisely diametrically sized to receive the shafttherewithin such that a controlled annular radial gap 24 is providedbetween the outer surface 26 of the shaft 12 and the innercircumferential surface 18 of the sleeve. The annular gap 24 axiallyextends along the length of the bearing sleeve and is axially closed offby inwardly projecting circumferential tabs 29 or may alternately beaxially retained by other mechanical features, depending on theparticular foil design. The corrugated foils 28 of the bearing aredisposed within the annular gap 24, and extend about the fullcircumference of the bearing sleeve. The foils 28 of the present bearingpreferably have an axial length substantially equal to the length of thegap 24, however several separate sections of corrugated foils, eachextending about the full circumference of the sleeve but axiallyextending only along a fraction of the full axial length of the bearingsleeve, could nevertheless be used. The corrugated foils 28 aregenerally fixed at an outer radial edge to the inner circumferentialsurface 18 of the bearing sleeve, and extend freely into the annular gapat inner radial edges thereof.

The foils act much as is well known in foil bearings of the prior art.As opposed to traditional journal bearings wherein the radial clearancebetween the shaft and the bearing sleeve is fixed, the foils of thefoil-fluid bearing of this invention deflect radially outward in orderto provide a relatively constant radial fluid film clearance when shaftgrowth due to temperature and centrifugal forces occurs. As the shaftturns, hydrodynamically generated pressure pushes the foils away fromthe shaft, keeping the shaft completely fluid-borne. When the shaftradially grows due to thermal expansion, the foils get pushed radiallyfurther away, keeping the fluid film clearance relatively constant.

While foils can be themselves made flexible in order to provide somedamping which is required to suppress whirl instability resulting frombearing misalignments, in order to be sufficiently compliant the priorart flexible foils must be considerably complex and are thereforeexpensive to manufacture. The present invention can use simple andrelatively inexpensive non-flexible foils, as damping to prevent whirlinstability and bearing misalignment is provided by the compliantbearing sleeve supports. As radial stiffness and the dampingcharacteristics of the bearing are provided by the compliant supports,externally from the inner foil construction of the bearing, significantmanufacturing cost reductions can be realised.

As the compliant supports can be removed and replaced with othercompliant elements having different stiffness, size or materialcharacteristics, the radial stiffness and misalignment tolerances of thepresent foil bearing can easily be modified as required for a specificapplication.

The compliant support arrangement of the present foil bearing permitssignificantly improved capability to tolerate angular misalignmentbetween radial bearings. This subsequently permits the use of bearinghousings that are less finely toleranced, resulting in significantmanufacturing cost savings.

A single central compliant support for the bearing sleeve could be usedin specific limited applications, however care must be taken to ensurethat such applications, having the present foil-fluid bearing with asingle compliant support, are running at a rotational speed outside thenatural frequency envelope of the bearing and shaft assembly, otherwiseexcessive oscillation of the sleeve can occur. The two compliant endsupports located either side of the central, and stiffer, compliantsupport provide damping against such natural frequency oscillation ofthe sleeve. Therefore, the three point compliant support arrangement ispreferable, however a single central compliant support for the bearingsleeve within the housing can nevertheless be used for applications thatwill always operate outside the above-mentioned rigid body modefrequency of the sleeve.

The total radial stiffness and damping characteristics of the preferredembodiment of the present compliant support arrangement, wherein astiffer central compliant bearing sleeve support is disposed between twocompliant end supports having a lower modulus of elasticity than thecentral support, can be controlled and varied as required by propercompliant material and size selection, as it would for two or morecompliant supports having equal stiffness. The rotational flexibility ofthe bearings sleeve about an axis perpendicular to the longitudinal axisof the shaft can also be varied as required, by changing the distancebetween the stiffer middle compliant support and the side compliantsupports.

The foil-fluid bearing of the present invention is preferably an airbearing, although it can equivalently be used for foil bearings using aprocess fluid other than air, such as helium, xenon, refrigerants,liquid oxygen and liquid nitrogen.

The present foil-air bearing could be used in a variety of applications,however it is particularly intended for use in an air cycle machine,employed in an aircraft air conditioning system for example. Almost allaircraft currently use air cycle machines in their environmental controlsystems, and the present foil-air bearing particularly lends itself foruse in such an air cycle machine. The present foil-air bearing is alsoparticularly intended for use in a gas turbine engine. Such a foil-airbearing could, however, equally be used in other rotating machineryapplications, such as motor driven compressors and other fuel driventurbomachines.

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 21. A foil-fluid bearing assembly for use in a housing, the assembly comprising: a bearing sleeve, disposed between a rotating shaft and the housing, such that an clearance gap, adapted for receiving a support fluid therein, is provided between the rotating shaft and the bearing sleeve; a foil element being disposed within the clearance gap; at least two independently-deflectable elastic supports supporting the bearing sleeve within the housing, disposed between the bearing sleeve and the housing; and the first elastic support being radially stiffer than the second elastic support.
 22. The foil-fluid bearing assembly as defined in claim 21, wherein the at least two elastic supports comprise three supports including a central support being stiffer than two opposing end supports.
 23. The foil-fluid bearing assembly as defined in claim 22, wherein the two opposing end supports have substantially the same stiffness.
 24. The foil-fluid bearing assembly as defined in claim 21, wherein the at least two elastic supports are disposed and retained in respective recesses in the bearing sleeve.
 25. The foil-fluid bearing assembly as defined in claim 21, wherein the at least two elastic supports are deflectable independently from foil element movement.
 26. The foil fluid bearing assembly as defined in claim 21, wherein the at least two elastic supports are contiguously disposed around an interface between the bearing sleeve and the housing.
 27. The foil fluid bearing assembly as defined in claim 21, wherein the at least two elastic supports and the bearing sleeve facilitate rotational flexibility of the bearing sleeve about an axis perpendicular to a longitudinal axis of the shaft.
 28. A foil-fluid bearing assembly adapted for rotatably supporting a shaft within a bearing housing, the foil-fluid bearing assembly comprising: a bearing sleeve having an outer surface and an inner surface defining an inner race, said inner race sized to receive a rotating shaft therewithin and maintain a substantially annular clearance gap between said rotating shaft and said inner surface, the bearing sleeve adapted, in use, to direct a pressurized support fluid to substantially fill said clearance gap and thereby rotatably support said shaft; a foil element circumferentially disposed between said inner race and said shaft; and a bearing support disposed between the outer surface of the bearing sleeve and the bearing housing, the bearing support having three independently-deflectable elastic support elements arranged with a central support element between two end support elements, said central support element being radially stiffer than the end support elements.
 29. The foil-fluid bearing assembly as defined in claim 28, wherein the central support element is substantially disposed at an axial midpoint of the bearing sleeve.
 30. The foil-fluid bearing assembly as defined in claim 28, wherein the central support element is disposed substantially equidistantly between the end support elements.
 31. The foil-fluid bearing assembly defined in claim 28, wherein the end support elements are disposed adjacent opposing ends of the bearing sleeve.
 32. The foil-fluid bearing assembly as defined in claim 28, wherein the end support elements have substantially the same stiffness.
 33. The foil-fluid bearing assembly as defined in claim 28, wherein the elastic support elements are disposed and retained in respective recesses in the bearing sleeve.
 34. The foil-fluid bearing assembly as defined in claim 28, wherein the elastic support elements are deflectable independently from foil element movement.
 35. The foil fluid bearing assembly as defined in claim 28, wherein the elastic support elements are contiguously disposed between the bearing sleeve and the bearing housing. 